The gills, skin, fins, head and gastrointestinal tract were not included in the measurement of influx, since these tissues contain a large fraction of Cd that is retained in the surface epithelium and has not been transferred to the circulatory system. Measurement of Cd in blood could not be used to estimate influx, since the Cd level is very low in blood due to rapid uptake from the blood by the internal organs. Excretion of Cd from the fish did not affect the influx results, since Cd elimination from the internal organs of zebrafish is very slow and cannot be detected for up to 83 days after a 24-h exposure to Cd